The present invention relates to an image forming apparatus such as color copiers and printers.
Conventionally, so-called tandem-type color laser printers have large advantage of being able to process image data including monochrome pages and color pages both at the same print speed because latent image formation and toner image formation are executed independently by each of colors Y (yellow), M (magenta), C (cyan) and K (black). In the case of printing a document including both monochrome and color pages, printing all the pages in a color print mode causes increased load or wear of a color processing section, and therefore it is preferable to switch between a color print mode and a monochrome print mode depending on each page.
However, depending of a mixture rate of color and monochrome pages, the print mode should be switched frequently and a time taken for this switchover increases as a loss time. With respect to determination of the print mode in which the monochrome pages should be printed, various technologies (see e.g., JP 2003-237189 A) aiming at reducing the loss time have conventionally been proposed.
In a paragraph 0049 of JP 2003-237189 A, print speeds for the monochrome mode and for the color mode are varied. However, the print speed is just the “print speed” and not necessarily refers to a system speed. Generally, the print speed is not uniquely determined by the system speed but is largely influenced by other elements such as sheet intervals, and it is known that changing such elements increase the print speed.
In the meanwhile, in the tandem-type image forming apparatus supporting monochrome and color printing, the monochrome print mode which does not need to drive a color processing section can increase the system speed and consequently increase the print speed proportionally because the power consumption of the color processing section can be allotted to other sections. However, in a document including both monochrome and color pages, if the print mode is switched depending on the monochrome/color identification of each page in the document, then the system speed should also be switched accordingly. In switching the system speed, the system speed cannot be changed until the latest sheet leaves a fixing device, and this waiting period is a print stop time during which image forming process cannot be started. In the case where frequency of switchover between monochrome and color pages in the document is high, the print stop time increases cumulatively every time the print mode is switched, and this decreases the printing number per time less than a specified value.